1. Field of the Invention
The present invention relates to an electronic circuit device used for vehicle AC regulators or the like, and more particularly, to a circuit for protecting electronic circuits in the device from a change in a ground potential when a ground terminal of the electronic circuit device has been removed.
2. Description of the Related Art
As an electronic circuit generally used for vehicle AC regulators or the like, a low-side switching system circuit arrangement has been widely employed. In this circuit, one terminal of an output switching element is connected to an external power supply through an external load and the other terminal of the output switching element is grounded.
FIG. 4 shows a low-side switching system electronic circuit whose output terminal is connected to an external power supply through an external load. More specifically, an input terminal 1 for receiving a signal supplied from an external circuit A is connected to an output driver circuit 2, and the output driver circuit 2 is connected to an output switching element 3. The output switching element 3 is connected to an output terminal 4 of the electronic circuit. The output driver circuit 2 is connected to an external power supply 6a through a power supply terminal 5. The output driver circuit 2 and the output switching element 3 are connected to a ground terminal 7 and grounded.
One terminal of an external load 8a is connected to the output terminal 4, and the other terminal of the load 8a is connected to the external power supply 6a.
In such an electronic circuit, the output switching element 3 is driven by a driving signal from the output driver circuit 2, and its output signal is fed from the output terminal 4.
FIG. 5 shows a high-side switching system electronic circuit, wherein an output switching element is connected to an external power supply, and an external load is grounded.
More specifically, as in FIG. 4, an input terminal 9 for receiving a signal from an external circuit A is connected to an output driver circuit 10, and the output driver circuit 10 is connected to an output switching element 11. The output switching element 11 is connected to an output terminal 12 of the electronic circuit.
The output driver circuit 10 and the output switching element 11 are connected to an external power supply 6b through a power supply terminal 13. One terminal of the output driver circuit 10 is connected to a ground terminal and grounded. One terminal of an external load 8b is connected to the output terminal 12, and the other terminal of the external load 8b is grounded.
In such a high-side switching system electronic circuit, since the external load is grounded as compared with the conventional low-side switching system electronic circuit, it can be easily replaced so that the electronic circuit is easily handled. According to this advantage, a low-side switching system electronic circuit used for vehicle AC regulators is being replaced with a high-side switching system electronic circuit.
However, in the high-side switching system electronic circuit shown in FIG. 5, when a ground terminal 14 has been removed from the ground, the safety associated with controlling the electronic circuit or the like is not considered.
More specifically, when the ground terminal 14 has been removed from the ground, a current from the external power supply 6b is supplied to the ground terminal 14 through the circuits constituting the electronic circuit, and an electric potential of the ground terminal is increased. The applied voltage from the external power supply 6b, which is measured from the ground terminal 14 whose electric potential is a reference electric potential, appears to be decreased by an increase in the electric potential of the ground terminal 14.
As a result, while the voltage of the external power supply 6b is applied to the output switching element 11, the input signal is supplied to it under such a condition that the reference voltage is changing, thereby feeding the output signal to the external load 8b through the output terminal 12. Therefore, the output of the electronic circuit will not be cut off while keeping the output control under the unstable condition. Accordingly, the output control will become impossible.